A variety of different implantable medical devices (IMD) are available for therapeutic stimulation of the heart and are well known in the art. For example, implantable cardioverter-defibrillators (ICDs) are used to treat those patients suffering from ventricular fibrillation, a chaotic heart rhythm that can quickly result in death if not corrected. In operation, the ICD continuously monitors the electrical activity of a patient's heart, detects ventricular fibrillation, and in response to that detection, delivers appropriate shocks to restore normal heart rhythm. Similarly, an automatic implantable defibrillator (AID) is available for therapeutic stimulation of the heart. In operation, an AID device detects ventricular fibrillation and delivers a non-synchronous high-voltage pulse to the heart through widely spaced electrodes located outside of the heart, thus mimicking transthoratic defibrillation. Yet another example of a prior art cardioverter includes the pacemaker/cardioverter/defibrillator (PCD) disclosed, for example, in U.S. Pat. No. 4,375,817 to Engle, et al. This device detects the onset of tachyarrhythmia and includes means to monitor or detect progression of the tachyarrhythmia so that progressively greater energy levels may be applied to the heart to interrupt a ventricular tachycardia or fibrillation. Numerous other, similar implantable medical devices, for example a programmable pacemaker, are further available.
Regardless of the exact construction and use, each of the above-described IMDs generally includes three primary components: a low-power control circuit, a high-power output circuit, and a power source. The control circuit monitors and determines various operating characteristics, such as, for example, rate, synchronization, pulse width and output voltage of heart stimulating pulses, as well as diagnostic functions such as monitoring the heart. Conversely, the high-power output circuit generates electrical stimulating pulses to be applied to the heart via one or more leads in response to signals from the control circuit.
The power source provides power to both the low-power control circuit and the high-power output circuit. As a point of reference, the power source is typically required to provide 10–20 microamps to the control circuit and a higher current to the output circuit. Depending upon the particular IMD application, the high-power output circuit may require a stimulation energy of as little as 0.1 Joules for pacemakers to as much as 40 Joules for implantable defibrillators.
Suitable power sources or batteries for IMD's are virtually always electrochemical in nature, commonly referred to as electrochemical cells. Acceptable electrochemical cells for IMDs typically include a case surrounding an anode, a separator, a cathode, and an electrolyte. The anode material is typically a lithium metal or, for rechargeable cells, a lithium ion containing body. Lithium batteries are generally regarded as acceptable power sources due in part to their high energy density and low self-discharge characteristics relative to other types of batteries. The cathode material is typically metal-based, such as silver vanadium oxide (SVO), manganese dioxide, etc.
IMDs have several unique power source requirements. IMDs demand a power source with most of the following general characteristics: very high reliability, highest possible energy density (i.e., small size), extremely low self-discharge rating (i.e., long shelf life), very high current capability, high operating voltage, and be hermetic (i.e., no gas or liquid venting).
These unique power source requirements pose varying battery design problems. For example, for the heart monitoring function of an AID, it is desirable to use the lowest possible voltage at which the circuits can operate reliably in order to conserve energy. This is typically in the order of 1.5–3.0 V. On the other hand, the output circuit works most efficiently with the highest possible battery voltage in order to produce firing voltages of up to about 750 V. Traditionally, all manufactured implantable cardioverter defibrillators used a battery system comprised of two cells in series to power the implantable device. This power source of approximately 6 volts provided improved energy efficiency of the output circuit at the expense of energy efficiency of the monitoring circuit. However, a two-cell battery was undesirable from a packaging, cost, and volumetric efficiency perspective.
Eventually, improvements in output circuit design allowed the use of a single 3-volt cell while still maintaining good energy efficiency. Most ICDs are now designed with a single cell battery instead of dual cells connected in series. This approach was taken to improve the volumetric efficiency. In order to achieve the same power capability of the dual cell approach, the electrode surface area of the single cell must be at least equivalent to the total electrode surface area of the dual cell battery. However, the increased electrode surface area of a single cell poses a potential hazard to the IMD should an internal short circuit develop in the battery cell. If the electrode surface area is too high (approximately above 90 cm2 for a Li/SVO battery) and an internal short develops, the battery can get hot enough to potentially destroy the IMDs electronics and possibly burn the patient. As a result, most IMD and IMD battery manufacturers have adopted a design rule, which limits the surface area of a single cell to approximately 90 cm2. This is significantly less surface area than a typical dual cell design where the surface area was approximately 130 cm2. Hence, these single cell ICD batteries produced less power and the result was longer capacitor charge times. Many studies have proposed that defibrillation and cardioversion shocks are most effective when delivered as quickly as possible following detection of arrhythmia. The chance of terminating an arrhythmia in a patient decreases as the length of time it takes for therapy to be delivered to the patient increases. Therefore, the shorter the charge time for the capacitors the more effective the defibrillation therapy. Typically, battery electrode sizes are inversely proportional to the charging time. Therefore, the quicker the desired charging time, the larger the battery.
While single battery systems have proved workable for implantable cardioverter defibrillators, the use of a single battery system necessarily involves a compromise between the ideal power supply and the hazards associated with large surface area electrodes. Accordingly, it would be desirable to provide for an improved dual battery power system for an implantable cardioverter defibrillator, which overcomes the problems of earlier attempts at dual battery systems.